1. Field of the Invention
This invention relates to the field of communications, and in particular to time-division-duplex (TDD) transceivers with a common transmit and receive frequency.
2. Description of Related Art
Time-division-duplex (TDD) transceivers are commonly used to provide two-way communications using a single carrier signal frequency. FIG. 1 illustrates an example block diagram of a conventional time-division-duplex transceiver 100 that utilizes quadrature modulation. The transceiver 100 includes a transmitter 130 that transforms an input data signal into quadrature signals TI 131 and TQ 132. A local oscillator 120 provides an in-phase oscillation signal 121, and a phase shifter 125 provides a quadrature-phase oscillation signal 122 that is 90 degrees out of phase with the in-phase oscillation signal 121. The quadrature signal TI 131 is modulated, at 142, by the in-phase oscillation signal 121, and the quadrature signal TQ 132 is modulated, at 144, by the quadrature-phase oscillation signal 122. The adder 150 combines these modulated signals to produce a composite signal 151.
The transmit/receive switch 160 alternately selects the composite signal 151 for transmission, via an antenna 165. On the alternate cycle, the transmit/receive switch 160 provides an input signal 161 from the antenna 165. Although an antenna 165 is illustrated in FIG. 1 (and FIG. 3), the use of other communications media, such as a wire, or cable, is also common in the art.
The input signal 161 is a composite signal that is segregated into corresponding quadrature signals RI 173 and RQ 175 by demodulators 172 and 174, respectively. Common in the art, the local oscillator 120 that is used to modulate the transmit quadrature signals TI and TQ is used to demodulate the received input signal 161 into receive quadrature signals RI and RQ. A number of advantages are achieved by using a common local oscillator 120. In particular, the local oscillator 120 is typically a phase-locked oscillator, and using the same oscillator 120 during both phases of the transmit/receive switch 160 eliminates the need to re-phase or re-synchronize the oscillator 120 with each transition. Additionally, the use of the same local oscillator 120 provides a material cost savings compared to the use of a separate oscillator for each of the transmit and receive operations. The receiver 110 processes the quadrature signals RI 173 and RQ 175 to provide an output signal 102.
As is common in the art, the transmitter 130 provides the transmit quadrature signals TI 131 and TQ 132 at a predetermined intermediate frequency (IF). In like manner, the quadrature signals RI 173 and RQ 175, being produced by a distant transmitter that is similar to the transmitter 130, are also produced at the predetermined intermediate frequency. The modulation 142, 144 of the quadrature signals TI 131, TQ 132 at the intermediate frequency IF with the local oscillation signals 121, 122 at a carrier frequency Fc results in two sidebands of modulation, one at Fc+IF, and the other at Fc−IF. Ideally, the quadrature signals TI 131 and TQ 132 are structured such that one of the sidebands, the intended sideband, contains maximum power, while the other sideband, the “image” sideband contains minimum power.
Due to component variations and other factors, however, a difference in phase or amplitude from the ideal relationship between the quadrature signals TI 131 and TQ 132 can result in an image sideband having a considerable power content. FIG. 2 illustrates an example spectral power density plot of a convention transmitter 130 having a less-than-ideal relationship of amplitude or phase between the quadrature signals TI and TQ. As illustrated, a majority of power is located at the intended sideband at Fc+IF, at 220, but a considerable amount of power is illustrated at the carrier frequency FC, at 210, and at the image sideband at Fc−IF, at 230. To minimize the distortion of the demodulated intended signal, the transmitter or a distant receiver must filter this unintended and undesirable carrier and image signal power.
As is known in the art, the cost and complexity of a filter process is highly dependent upon the degree of “roll-off” required of the filter. The selective filtering of two signals that are close in frequency requires a very steep roll-off, and therefore is more costly and complex than the selective filter of two signals that are widely separated in frequency. By implication, then, the preferred intermediate frequency IF should be large, because the separation between the intended 220 and unwanted 230 signals is twice the intermediate frequency. However, a high intermediate frequency introduces additional costs and complexities to the components utilized within the transmitter 130 and receiver 110 compared to a lower intermediate frequency. Preferably, the transmitter 130 should be designed to conform as close to the ideal as possible, so that the degree of required filtering at the transmitter or distant receiver can be minimized, and so that a lower intermediate frequency can be utilized. The use of precision components and robust design techniques that provide for this idealized transmitter performance, however, is also a costly and complex approach.